Frame design

After building the frame in Blender3d, I have a real good idea of how it needs to look.  However, my 3d rendering is not to scale.  So I have to do a 2d side view of the frame to determine it’s proper dimensions.

For  the people that like “plans”, this is about as close as I get.   If the dimensions aren’t perfect, I can make up for it by moving the bogies (and roller wheels) up or down until everything is about the right shape.

So, I used my drawing from TrackMath, and modified it to figure out the frames dimensions.  I did make one slight change from  the 3d model, which was having a small flat part in the back, instead of a straight 45-degrees to the top rail.

The frame is of course, the thick blue line.   That’s pretty much that.  I know, nothing ground-breaking here, but a necessary step nevertheless.   And most importantly.. it means I can begin the frame construction soon!

I’m going to use square tube, probably like 3/4″ OD.  I will decide for sure once I feel the actual material, and get an idea of weight.

I also need to decide exactly how wide to make the frame.   I can’t think of a good way to do that, other than pretty much winging it.  Total width will be around 3ft, but this frame will fit inside the tracks, so about 2ft wide… I guess.

Motors, gearing, and speed.

I have done some very preliminary research into what I should use for the electric motors.   Thanks to the wonderful popularity of those silly motorized scooters, there actually a decent number of affordable electric motors.  I will need 2 motors of course, one for each side.  This way each track can go forward or reverse, independently of the other.

The motors I’m considering at this point are 24v, around 250W and run up to like 2500 rpm.  They even come with small pulleys, around 1″, already on them  (I will have a future post about belts and pulleys versus chains and sprockets).

So, I wanted to get an idea of the top speed the tank would have, if the motors were running at 2500 rpm.   I am figuring a gear ratio of 6:1 (1″ pulley on the motors, to a 6″ pulley on the drive wheels).  I will also have approximately 6″ drive wheels, which, when driving a track, you can pretend they are actually just wheels on the ground, and the numbers will work out close enough (there is a slight difference due to the thickness of the track, but that’s not a huge deal for now).

Anyways, here’s the equation to figure out top speed:

(RPM/gear ratio)  x  60  x  WheelDiameterInches x Pi
———————————————————————-
5,280 x 12
= 7.4 mph with a 1:6 gear ratio
=5.4 mph with a 1:8 gear ratio

I am very happy with those speed estimates.  If they came out to like 30 mph, I would know I’d need to gear it way down or get lower revolution motors… likewise if it came out to 1 mph I would need to gear it up and so on.   Also, when finally selecting the motors, if it’s a high torque motor, but turns way less revolutions, I will know I can use more even gearing, etc etc.

Installing the motors is obviously far off at this point, but these things need to be considered.  For instance, I am leaving myself enough room for a 6″ pulley on the drive wheels, when I construct the frame, axles, and the other associated parts.    That’s about it for today, sorry, no cool pictures, but at least we did some handy math you can use when making your own tank.

UHM…W, I call it. UHMW the wonder plastic.

I have decided to use UHMW, which is a hard plastic, to make the drive sprockets and track treads out of.  And other stuff later on.   You can get remnant/scrap pieces, ($1 per pound in my case), which is a very fair price… after all, it’s plastic, not lead.

Why UHMW?   It is a thermoplastic polyethylene.   It has the “highest impact strength of any thermoplastic presently made.”   It has very low friction (less than nylon and acetal, and is comparable to Teflon), is self-lubricating, and is highly resistant to abrasion. It is odorless, tasteless, and nontoxic.  It is also very resistant to water, moisture, most chemicals, UV radiation, and micro-organisms. *

Basically it’s super tough.   BUT, the other great feature, is you can cut it with most woodworking tools… about the only thing you don’t wanna do is sand it with sandpaper.    I was also able to get it in black, which is nice, will give the tracks a nice realistic look.

I got 1/2″ thick for the treads.  I decided I will cut them 1″ wide, and I think the 1/2″ will be perfect… I had originally said 1″ thick, but I think that is overkill… it would weigh twice as much, and would dig or catch more into soft ground, which should not be necessary.

For now, I cut out one drive sprocket, just need too clean up the edges a little:

* – Copied verbatim or paraphrased from http://en.wikipedia.org/wiki/Ultra_high_molecular_weight_polyethylene

Track treads, and some 3d

Ok, I did a litte math to get an idea of how many treads I will need on the conveyor belts to make the tracks.

My drive sprocket is 8 tooth (thus, 8 valley’s or, the part between the teeth).   Diameter of the drive sprocket is 6″, so the circumference is:  18.84

So every 18.84 inches along the track, there needs to be 8 treads.

The track is 129″ long, so, to figure the number of treads per track, setup a little ratio/cross multiply:

8                    X
——–   = ———
18.84          129

X = 54 treads per track

So, I need 108 treads total.  not too bad I guess.   As long as making each one doesn’t require too many steps.

Ok, now for the fun stuff.  Finally did some 3d modeling in Blender3d (it’s a very slick, free program, look it up), as you can see from the new main page image.

I plan on building the body in 2 stages.  First, the tub, and rolling chasis.  Then a front nose piece and the rest of the body.  Something like this, do enjoy:

Scale: 1/4th

So I have finally determined the tank will be approximately 1/4 scale.   An M4 Sherman tank is a little over 19ft long, mine will be almost 5ft.. which is obviously about 1/4.

How I determined this?   I have 2 pieces of conveyor belt, 129″ long… one for each track.  My drive wheels and roller wheels are 5-1/8″.   I started drawing with a relative scale in MSPaint, decided to have the track come up at a 45° in the front and back, and the rest just sorta fell into place.   One cool part was determining the length of where the track wraps around the wheels in a circular fashion…. basically a trapezoid with rounded corners.

If you are into the math, it’s kinda cool, but also shows the different dimensions (note: length is not drawn to scale as it was the variable when I was figuring this out):

To be to the scale of a real Sherman, it would need to be only a little over 2 feet wide, but I will probably go a little bit wider, like 32-36″.  Now that I have some pretty decent dimensions figured out, I can start doing some 3d-modeling in Blender3d.   This will help a lot of my decisions down the road, such as building the lower pan, bogies, etc.

I am also currently looking at a few different options for the tracks treads.  The 2 finalists at this point are composite decking material, cut into slices on the table saw, or UHMV, a hard plastic, that is very versatile, and would hold up really well, but may be a little harder to come by at a reasonable price. Time will tell… but at least now I know what I’m building… a 1/4 Scale Sherman Tank.

Moron err, more on drive wheels…

One thing I glossed over was how I made my drive sprockets pattern.  I guess there are probably lots of ways to do this, but the way I did it is simple, and very accurate.  It’s main drawback being the number of teeth will always be 2^n, or(4, 8, 16, 32) … but, for me, an 8 tooth works good.

Here’s how I did it:

Looks like this:

If you read yesterdays post about Drive Wheels, but didn’t quite get how it all lines up, I made a cross section that may help to make sense of a few things… like how the belt sits on the plastic wheels, how I plan to connect the treads to the belt, and a sneak preview of how I plan on keeping the track from coming off … with the inside alignment blocks, which I think have a real name, but that’s what I call them.  So, without further ado, more MSPaint magic:

Drive Wheels from scratch

For my drive wheels, I want them to use some drive sprockets that sit on the outside of the conveyor belt (if you’re confused, read yesterdays post about Tracks).   But I figure why not also use some friction to my advantage, so I decided to have the conveyor belt roll along some rubber wheels (similar to those on the front of a lawn mower) , while at the same time have the drive sprockets push my overhanging treads.

I started with 2 floor flanges for 3/4″ pipe, and a 3″ length of threaded pipe:

hub

Then I use 4 bolts while I weld it to make sure everything’s nice and square:

After welding it up, I have some of the plastic/rubber wheels, drill aligning holes, and will eventually drill the same holes into the drive sprockets (the wood things).  These are 2 pieces of 5/8″ plywood glued together for now. They should hold up ok, if not, I will remake them out of something stronger later.  Here are a couple of pics of what it should look like when assembled (still need to finish the other drive sprocket on the bottom):

Finally, there will be a 5/8″ axle or long bolt going through the drive wheels.  This will either be a live half-axle, or fixed and the drive wheels will have their own bearings and have a pulley on the end.  I will deal with that later, much bigger fish to fry for the time being.